1. Field of the Invention
This invention relates to glycolipid esters and in particular to a process for producing a glycolipid ester having surface activity and wax-like properties and represented by formula (I), ##STR2## wherein R.sub.3 represents a hydrogen atom or a methyl group, R.sub.4 represents a saturated or unsaturated hydrocarbon group having 12 to 16 carbon atoms when R.sub.3 is hydrogen, R.sub.4 represents a saturated or unsaturated hydrocarbon group having 11 to 15 carbon atoms when R.sub.3 is methyl, and R represents a saturated or unsaturated alkyl group having 2 to 20 carbon atoms.
2. Description of the Prior Art
Higher fatty acid esters of sucrose: (sugar esters) and higher fatty acid esters of anhydrosorbitol (Span) have been widely used as surface active agents, particularly in the field of emulsifying agents. Such known esters are formed via the ester bonding between the hydroxy group of the sugar moiety and the higher fatty acid, and the surface activities of these esters are regulated according to their ester values. Selective esterification of sugar at its desired position or positions is nearly impossible because many hydroxy groups exist in the sugar structure. In this situation, only mixtures of various ester isomers are utilized on an industrial basis. The hydrophilic properties of these esters are significantly decreased as the ester values increase because the esters are formed by esterification of the hydroxy groups in the sugar. Consequently, much difficulty is encountered with the emulsifying processes. Another problem is that the esters are chemically unstable by reason of the fact that the ester bonds of the hydroxy groups in the sugar and higher fatty acid are more easily hydrolyzed than those of the common fatty alcohol esters.
It has been reported by J. F. T. Spencer et al [Canadian Journal of Chemistry, 39, 846 (1961)] that great quantities of Sophorolipid are produced by culturing Torulopsis bombicola.
Sophorolipid is a mixture of the compounds represented by formulas (IIa) and (IIb), ##STR3## IIa-1: R.sub.1 =R.sub.2 =COCH.sub.3 IIa- 2: R.sub.1 =COCH.sub.3, R.sub.2 =H
IIa-3: R.sub.1 =H, R.sub.2 =COCH.sub.3 PA1 IIa-4: R.sub.1 =R.sub.2 =H PA1 IIb-1: R.sub.1 =R.sub.2 =COCH.sub.3 PA1 IIb-2: R.sub.1 =COCH.sub.3, R.sub.2 =H PA1 IIb-3: R.sub.1 =H, R.sub.2 =COCH.sub.3 PA1 IIb-4: R.sub.1 =R.sub.2 =H
wherein R.sub.3 and R.sub.4 in formulas (IIa) and (IIb) are the same as defined above.
As can be seen from formulas (IIa) and (IIb), Sophoropilid is a mixture of many glycolipids, and its basic structure is of a [(2'-O-.beta.-D-glycopyranosyl-.beta.-D-glycopyranosyl)-oxy]-alkane acid or alkene acid which is obtained via the glycoside bonding between Sophorose and a long-chain fatty acid having a hydroxy group at the .omega. or .omega.-1 position.
The compound of the present invention possesses structural features which cannot be found in the conventional glycolipidtype surface active agents and which are characterized by the fact that a stable glycoside bond is formed by the hydroxyfatty acid and sugar and that the end group of the alkyl or alkenyl group is a reactive carboxyl group. The compound has a greater chemical stability than those containing the conventional ester bonds because the hydrophobic alkyl or alkenyl group is linked to the hydrophilic group or sugar via glycoside bonding. Moreover, the alkyl or alkenyl group which has hydrophobic properties, is occupied at its end by the reactive carboxyl group, and it is possible to produce glycolipids possessing surface activities and wax-like properties which have wide application by modification of the carboxyl groups only while leaving unmodified the hydroxy groups of the sugar moiety.
However, the production of the compound of formula (I) from Sophorolipid by fermentation involves some problems. Namely, Sophorolipid cannot be used as a starting material because it is a mixture of many homologs having a lactone ring, a free carboxyl group, acetyl groups and the like as shown in formulas (IIa) and (IIb). Accordingly, compound of formula (IIb-4) should be first produced by eliminating the acetyl groups and releasing the carboxyl group without destruction of the carbon framework. The compound of formula (IIb-4) is a highly viscous substance peculiar to a sugar compound and is very difficult to obtain by means of conventional methods.
When Sophorolipid is forcibly dispersed in water, and an acid or alkali is added to the resulting suspension in an amount necessary for normal hydrolysis of the ester bond, part of the deacetylated or deacylated compound, which is subject to partial hydrolysis, acts as an emulsifying agent by incorporating the unreacted substances into micelles against continued attack by the remaining acid or alkali, thereby resulting in incomplete hydrolytic action. For instance, the reaction proceeds to an extent of only about 50%, even if a given amount of potassium hydroxide (0.25 part per one part of Sophorolipid) is added to an aqueous solution containing 20% of Sophorolipid, and the resulting solution is hydrolyzed with heating for 6 hours. When hydrochloric acid is used in an amount of 5% instead of the alkali catalyst, hydrolysis is as incomplete as in the case where the alkali catalyst is employed, and partial cleavage of the glycosyl ether bond is caused as well as damage to the basic structure.
If the reaction is completed under the above conditions, potassium hydroxide should be used in an amount of 0.25 part per one part of Sophorolipid, which is an extremely great amount of base which is uneconomical, and it is nearly impossible to separate the compound of the formula (IIb-4) which forms from the reaction solution by any industrially acceptable process. One reason is that the compound of formula (IIb-4) is readily soluble in water, and can be only dissolved in lower alcohols such as methanol and ethanol or special expensive organic solvents such as pyridine, dimethylsulfoxide or dimethylformamide which would otherwise create a serious obstacle to safety. As another reason, such compounds have a viscosity of more than 100,000 cps at room temperature. Since potassium acetate which simultaneously forms is also readily soluble in water and alcohols, it is necessary to forcibly eliminate water and extract the potassium acetate with any one of those nitrogen-containing solvents, and subsequently distill off the solvent so that the compound of the formula (IIb-4) may be separated. This process is unacceptable from an industrial standpoint. It is not impossible but very difficult to obtain the compound of the present invention merely by reacting the free carboxyl group of the compound of the formula (IIb-4) with methanol because this compound is highly viscous. In addition, the glycosyl ether bond is cleaved under strongly acidic conditions since there is no proper solvent capable of dissolving the compound.
With the above-noted difficulties in mind, studies have been conducted and it has been found that an hydrous Sophorolipid of a low viscosity can be obtained by adding a small amount of at least one polyhydric alcohol represented by formulas (III) or (IV), ##STR4## wherein R.sub.5 represents a hydrogen atom or a methyl group, R.sub.6 and R.sub.7 represent a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and m and n represent integers from 1 to 6, to hydrated Sophorolipid which is a fermentation product of Torulopsis bombicola, and distilling off water under reduced pressure by application of heat. Such finding is disclosed in a co-pending application Ser. No. 928,964.
The present inventors have made an attempt to subject the above noted Sophorolipid of a lower viscosity to alcoholysis to deacetylate and cleave its lactone ring, and at the same time, esterify the free carboxyl group. As a result of this attempt, it has been found that the reaction is very slow with the use of an alcohol having 2 or more carbon atoms and, that, for instance, ethanol requires an extended period of reaction time by a factor of approximately 110 times that of methanol, although methanolysis and methylation proceed rather rapidly to yield a compound wherein R in the formula (I) is a methyl group. In the reaction over such a prolonged period of time, acid attacks and cleaves the glycoside bond, thereby resulting in damaged basic structure. The use of a long-chain alcohol forms its acetate having a high boiling point by alcoholysis, which is distilled off with much more difficulty than methyl acetate and cannot be separated successfully by any other fractional method. Thus, such a prior art method cannot produce a highly pure compound of the formula (I) on an industrial basis.
The present inventors have made continued studies on the solution of the above difficulties and have found that the compound of formula (I) can be obtained in a high purity and in high yield by producing, from Sophorolipid, methyl-[(2'-O-.beta.-D-glucopyranosyl-.beta.-D-glucopyranosyl)oxy]-alkanoa te and -alkenoate of the formula (I) wherein R is a methyl group, and subjecting the resulting compound to an ester interchange reaction with an alcohol having 2 to 20 carbon atoms.